The present invention relates to a thermostat; and more particularly, it relates to a thermostat which includes heat anticipation.
It is well known that if a conventional room thermostat with a bimetal actuator were to open its contacts only when the mechanical setting (i.e., the temperature at which the thermostat is set) is reached, the actual temperature of the room would rise well above the mechanical setting and even cause the room to become uncomfortably warm. The reason for this is that when the room temperature reaches the mechanical setting, there is still considerable heat stored in the furnace and duct work, and the blower will continue to deliver this heat to the room after the fuel is shut off. The problem of room temperature overshoot would be even further accentuated because of thermal delays between the ambient temperature of the room being heated and the temperature-sensing element in the thermostat. In other words, the actual temperature-sensing element would reach the mechanical setting long after the temperature of the room reached the mechanical setting. Thus, this delay would cause the room temperature to overshoot even further than that which would be caused only by heat storage in the heating system.
This problem is obviated in a conventional thermostat by incorporating a small heater (called the "heat anticipation resistor") adjacent the bimetallic element which generates heat inside the thermostat casing which is conducted directly to the bimetallic element, causing it to switch at the mechanical setting sooner than it otherwise would have switched.
The maximum heat rise of the sensor element due to anticipation heat alone is referred to as the "droop". It is normally of the order of 4.degree. to 5.degree. F. That is to say, if room temperature is at 70.degree. F. and the heat anticipation resistor is continuously energized, but the furnace is not turned on, the temperature of the sensing element will rise approximately four degrees.
The sensing element normally opens at a slightly higher temperature (upper trip point) than the temperature at which it closes (lower trip point). The two trip points thus define a hysteresis effect. The trip points vary with the mechanical setting of the thermostat.
In a conventional thermostat of this type, with the room temperature equal to the mechanical setting, the trip points are equidistant from the limits of the droop range and the thermostat will have a duty cycle of approximately fifty percent. That is, the contacts will be closed fifty percent of the time and open fifty percent of the time (assuming no change in room temperature). Further, the thermostat will go through a complete cycle four to six times per hour.
A small change in the room temperature will change the duty cycle of the thermostat contacts over a wide range. That is, for a small decrease in room temperature, the "on" time (when the thermostat contacts are closed) will increase. If the room temperature changes more than one-half of the droop, the thermostat locks on. That is, anticipation heat alone will not cause the contacts to open, the room temperature must also increase.
It would be desirable to have a thermostat with a solid state sensor element as the temperature sensor, doing away with the bimetallic element of current mechanical thermostats. In addition to the low cost and reliability of solid state circuit elements, such a thermostat could be used in controlling the heating or cooling of individual rooms or spaces in a large building as a function of the time of day. For example, set points could be stored in a microprocessor, and changed at different times of the day, taking into account the position of the sun or other environmental factors. However, a simple on/off type of electrical sensor would not have the desirable characteristics of a mechanical thermostat equipped with heat anticipation, as described above. The embodiments of the present invention are suitable for such systems as well as for direct replacement of conventional mechanical thermostats.
According to the present invention, a solid state temperature sensing element generates a signal representative of the temperature of the room. A second signal, called a set point signal, is representative of a desired temperature setting for that room. The two signals in digital form, are fed into a data processor, in the form of a microprocessor.
In the operation of one embodiment, when the temperature signal rises above the set point, the set point signal is instantaneously decremented to a lower fixed value which is referred to as the lower limit of a set point range and then periodically increased by fixed increments over a period of time until it eventually becomes greater than the temperature signal. When this occurs, the set point signal is instantaneously increased to a higher fixed value which, in the illustrated embodiment, corresponds to the upper limit of the set point range, and thereafter decreased periodically by fixed increments over a longer period of time. Thus, the temperature signal is compared with a composite signal which varies in time under control of the processor.
The duty cycle (which is defined as the portion of a cycle in which the control system generates an "ON" signal) depends upon the average temperature signal in relation to the midpoint of the predetermined set point range of the system for a given set point value. If the average temperature is below the set point, the "on" time of the cycle increases, whereas if the average temperature rises above the set point, the "on" time of each cycle decreases. If the temperature signal is above or below the set point range, the controller will accordingly be continuously turned "OFF" or "ON" respectively for as long as the condition persists.
In an alternative embodiment, the set point signal remains fixed (but adjustable) and the temperature signal is modified by the central processor to form a composite sensor signal which is incrementally increased to an upper fixed value (which may correspond to the upper limit of what is, in this embodiment, referred to as the composite sensor signal range) when it becomes greater than the fixed set point signal and thereafter periodically decreased by smaller increments. Correspondingly, when the composite sensor signal falls below the fixed set point signal, it is decreased to a lower fixed value (which may correspond with the lower limit of the composite sensor signal range) instantaneously and thereafter periodically increased by smaller increments.
The present invention thus provides an electronic thermostat capable of using solid state temperature sensors and which exhibits the desirable heat anticipation features of present mechanical thermostats, yet which is suitable for use with a time-of-day programmer.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.